Collation shrink films are films that are wrapped around an object to be packaged and shrunk to keep the units within the object together. The most common use of these films is in the packaging of multiple containers, such as bottles or cans which might contain food, beverages and so on. The collation shrink film is wrapped around a number of the containers, perhaps a 6-pack of drinks or 24 pack of food cans held in a cardboard base and shrunk around the containers. The skilled person is familiar with this rapidly expanding film area.
The wrapping process typically involves a shrink oven or shrink tunnel in which the film and object covered by the film is briefly heated to cause the collation shrink wrapping to occur. The plastic film then collapses around the multiple containers and holds the units in place.
Films which are used as collation shrink films obviously need to possess certain properties to make them commercially interesting. Some of the main characteristics that are required for film applications in this market segment are good shrinkage. The films must possess excellent strength after shrinkage, often referred to as load retention resistance. This property requires that films are stiff.
Films must resist puncture and must not be sticky. The consumer does not want the packaged product to stick to the film. The collation shrink films are cut during the wrapping process so the ability to be cut is important.
Sealing properties are also important. During the wrapping process, the two sides of the film are passed around the object being wrapped and are contacted, typically underneath the product being wrapped. These two film edges must be sealed and the seal strength needs to be high. The whole packaged ensemble is often carried simply by grabbing hold of the packaging film. If the seal strength is weak, the film can fail during this operation. The seal strength must ideally be strong enough to hold the weight of the object packaged. Where the object is a 24pack of tins, for example, the weight can be significant.
Finally, the brand owner of the goods being packaged ideally wants his products to be clearly visible through the packaging. Optical properties such as low haze and high gloss that result in a brilliant print appearance are important.
Low Density Polyethylene (LDPE) currently dominates the collation films market segment with its good shrink behaviour, especially in transverse direction (TD). It is known however, that multimodal LLDPE exhibits significant benefits over LDPE when blended with other linear low density polyethylene and high density polyethylene components. This mixture offers the highest performance on all key shrink parameters for collation shrink films.
Thus, current collation shrink film solutions are polyethylene blown films which are made from LDPE and LLDPE and/or HDPE. The LDPE is necessary to give high shrink rate and the LLDPE / HDPE component gives a combination of stiffness, toughness and bundling force (also known as cold shrink force).
A major consideration in this rapidly expanding market is costs. Film down-gauging, i.e. using thinner films offers an obvious way of saving costs as thinner films means less film per unit packaged. Down gauged films can only be used however, when the other properties of the film as discussed above are maintained. The mechanical properties in particular need to be maintained.
The present invention targets new collation shrink films that can provide these core properties but preferably down gauged. Today there are polyethylene based blown films for collation shrink applications in use with a thickness of approximately 38-45μm. The present inventors sought films having a thickness (before shrinkage) of less than 38 microns. Ideally, the present inventors sought to down gauge the films to approximately 28 to 32 μm.
One possible route to down gauged films might be to add high density polyethylene (HDPE). The high stiffness of HDPE gives the shrunk film the necessary bundling force, but it makes it necessary to use a higher temperature in the shrink tunnel to effect shrinkage. Moreover, the mechanical strength of the film is poor and the film breaks under stress (e.g. when the bundle is lifted or moved). This solution is not therefore favoured.
Surprisingly, the inventors have found that the use of uniaxially oriented multimodal LLDPE films offers a possible solution to the problem here. When using a monoaxial stretched film of multimodal based LLDPE it is possible to achieve a thin film providing high mechanical strength and high bundling force and at the same time a very high shrink rate/shrink force at significantly lower temperature compared to non oriented blown film.
It is a key feature of this invention that the temperature required to effect shrinkage is much lower than is conventionally used in a collation film shrink tunnel. Lower temperatures mean lower costs and the product being packaged is not exposed to as high temperatures. This minimises the risk of product degradation.
However, the use of uniaxial (machine direction) oriented films and the use of low temperatures during the collation shrink process is limited by the transverse direction shrink of these films. If there is insufficient transverse direction shrinkage then the bundling force of the film can be too low in the transverse direction. Goods packaged using such a film might therefore work their way loose during transport.
The inventors therefore propose the use of a binding tape or binding strip which is also a machine direction uniaxially oriented film, ideally of the same composition as the collation shrink film. This tape can be passed around the object being wrapped in such a way that when it is subjected to heat, the binding tape shrinks in its machine direction around the object. The tape is oriented such that its shrinkage provides bundling force in the transverse direction of the main collation shrink film. The tape needed can simply be cut from the main collation shrink film. The tape might also form a handle for carrying the goods.
It will be appreciated that in any collation shrink wrapping process two ends of the object being wrapped remain open. This provides a potential source of weakness in the collation shrunk wrapped object. A further benefit of the use of a binding tape is that this tape will at least partially cover the open end providing additional strength to the package and possibly a kind of handle.
In this way, an object can be shrink wrapped using a potentially down gauged machine direction oriented (MDO) film at lower temperature than conventional collation shrink films and high bundling forces. This is advantageous in terms of cost and due to the fact that the materials packaged are not subjected to such high temperature.